The present invention generally relates to a tuner for use in a telecommunication receiver, such as a television receiver or radio receiver. More particularly, the present invention relates to a tuner of a type having a fine tuning mechanism.
Most tuners currently employed in television receivers have a fine tuning mechanism for fine adjustment of the television receiver circuitry to an operating frequency within a selected television channel. In general, these tuners comprise a plurality of oscillation coils, mounted peripherally on a turret drum rotatable through any one of a plurality of detent positions corresponding in number to the number of the oscillation coils, and a fine tuning member for each oscillation coil movably or adjustably supported inside the oscillation coil for varying the inductance of the selected oscillation coil in relation to the local oscillator frequency.
In these tuners, various linkage mechanisms have been used for effecting selection of any one of the television channels and subsequent fine tuning after one of the television channels has been selected. Two of the prior art linkage mechanisms, which appear to be pertinent to the present invention, are illustrated in FIGS. 1 and 2, respectively, of the accompanying drawings and will now be described with reference thereto.
Referring first to FIG. 1, the prior art tuner comprises a turret drum, a portion of which is shown at 10, having a plurality of oscillation coils, one of which is shown at 11, which coils are peripherally mounted on said turret drum 10 in equally spaced relation to each other and also to the longitudinal axis of the drum 10. A fine tuning member 12 in the form of a bolt member having its head portion shaped to provide a gear 12a is employed for each oscillatory coil 11 and is supported by the turret drum 10 for adjustable or threading movement inside of the coil 11 for varying the inductance of the coil 11 as is well known to those skilled in the art.
The turret drum 10 is rigidly mounted on a channel selection shaft 13 rotatably extending outwardly through a wall member 14 forming a part of, for example, a chassis, it being understood that, incident to rotation of the channel selection shaft 13, the turret drum 10 can be stepwisely rotated to any one of a plurality of detent positions defined by any known detent mechanism (not shown) and being substantially equal in number to the number of the coils 11.
A fine tuning mechanism comprises a fine tuning shaft 15 coaxially mounted on the channel selection shaft 13 on one side of the wall member 14 remote from the turret drum 10 for rotary movement about and independently of the channel selection shaft 13 and also for axial sliding movement between projected and retracted positions in a direction parallel to the shaft 13. This fine tuning shaft 15 is normally biased to the projected position by a spring element 16, for example, a compression spring mounted on the shaft 13 and positioned between the wall member 14 and the fine tuning shaft 15.
At one end of the fine tuning shaft 15 adjacent the wall member 14, the shaft 15 has a radially outwardly enlarged portion 15a thereon, the outer periphery of said enlarged portion 15a having a drive gear 17 thereon.
The fine tuning mechanism further comprises a transmission gear assembly generally indicated at 18 and composed of a pair of gears 19 and 20 rigidly mounted on a common spindle 21 which loosely extends through a slot 14a in the wall member 14 and which is rotatably carried by a support member 22. The support member 22 is shown as being in the form of a substantially rectangular flat plate 23 having a lateral projection 23a of substantially right-angled triangular shape on one end protruding in a direction away from the wall member 14 and into the hollow interior of the enlarged portion 15a of the fine tuning shaft 15. The support plate 23 is supported by the wall member 14 by means of any known guide (not shown) for movement between engaged and disengaged positions in a direction perpendicular to the longitudinal axis of the channel selection shaft 13 and is normally biased to the disengaged position by the action of a spring element 24, for example, a tension spring connected between the other end of the support plate 23 and a portion of the wall member 14. Because the support plate 23 is biased to the disengaged position, the lateral projection 23a protruding into the hollow of the enlarged portion 15a of the fine tuning shaft 15 has the sloped surface constantly engaged with the inner peripheral edge of the enlarged portion 15a as shown and the gears 19 and 20, carried by the support plate 23 on the common spindle 21 rotatably inserted through said support plate 23 are respectively disengaged from the gears 17 and 12a.
With the tuner shown in FIG. 1 being constructed as hereinabove described, the fine tuning can be carried out by first axially pushing the shaft 15 towards the retracted position against the force of the spring element 16 and then rotating the shaft 15 in either direction about the longitudinal axis of the shaft 13 while the shaft 15 is still maintained in the retracted position. In particular, since the fine tuning shaft 15 is axially pushed from the projected position towards the retracted position against the force of the spring element 16, the inner peripheral edge of the enlarged portion 15a slides along the sloped surface of the lateral projection 23a, thereby causing the support plate 23 to shift from the disengaged position towards the engaged position against the force of the spring element 24. Upon completion of the movement of the shaft 15 to the retracted position against the force of the spring element 16, the support plate 23 is held in the engaged position with the gears 19 and 20 respectively engaged with the gears 17 and 12a. Therefore, it is clear that rotation of the shaft 15 while the latter is still held in the retracted position will be transmitted to the fine tuning member 12 through the transmission gear assembly 18.
Referring now to FIG. 2, while some components of the tuner shown in FIG. 2 which are structurally and functionally similar to those employed in the tuner shown in FIG. 1 are designated by like reference numerals used in FIG. 1, there is a substantial difference in construction between the tuner shown in FIG. 1 and that shown in FIG. 2, particularly in the transmission gear assembly 18 and the support for such assembly 18.
In FIG. 2, the end of the fine tuning shaft 15 adjacent the wall member 14 is radially outwardly enlarged to define a boss 15'a and the gear 17 is rigidly mounted on or integrally formed with the shaft 15 at the junction between the shaft 15 and the boss 15'a. The transmission gear assembly includes the spindle 21, having one end pivotally and rotatably supported by an overhanging wall member 14' secured to the wall member 14, and the other end loosely extending through a slot 14b in the wall member 14 and having a gear 20' rigidly mounted thereon. The transmission gear assembly 18 further includes a gear 19', rigidly mounted on a portion of the spindle 21 adjacent the overhanging wall member 14' and held in constantly meshed relation to the gear 17 on the shaft 15, and a conical block 25 coaxially and rigidly mounted on the spindle 21 between the gears 19' and 20'. The slot 14b extends in a direction perpendicular to the longitudinal axis of the channel selection shaft 13 and the spindle 21 is guided by the slot 14b to pivot between engaged and disengaged positions. However, the spindle 21 is normally biased to the disengaged position by the action of a wire spring 26, mounted over the spindle 21 and in contact therewith, and the gears 17 and 19' are so designed that no disengagement takes place even if the plane of either one of the gears 17 and 19' deviates from the plane of the other of the gears 17 and 19'.
With the tuner shown in FIG. 2 constructed as hereinabove described, an axial push of the shaft 15 towards the retracted position against the force of the spring element 16 causes the spindle 21 to pivot towards the engaged position due to the outer peripheral edge of the boss 15'a on the shaft 15 being slidingly engaged with the tapering surface of the conical block 25 on the spindle 21. Upon completion of the movement of the shaft 15 to the retracted position, the spindle 21 is pivoted to the engaged position with the gear 20' operatively engaged with the gear 12a and, therefore, subsequent rotation of the shaft 15 while the latter is still held in the retracted position is transmitted to the fine tuning member 12 in a substantially similar manner as hereinbefore described with reference to FIG. 1.
In the prior art tuner of the construction shown in FIG. 1 and described above, in order for the transmission gear assembly 18 to be supported by the support member 22, at least one of the gears 19 or 20 must be a member separate from the spindle 21. In other words, both of the gears 19 and 20 cannot be integrally formed with the spindle 21 and, therefore, when the gear 19 or 20 which is separate from the spindle 21 is to be rigidly connected to the spindle 21 after the latter has been inserted through the support plate 23, a time-consuming and costly connecting procedure, such as by the use of a welding or pressure-fitting technique, is required. In addition, because of the sliding engagement taking place between the sloped surface of the lateral projection 23a and the inner peripheral edge of the enlarged portion 15a of the fine tuning shaft 15 during the fine tuning operation, the durability of the tuner as a whole tends to be reduced.
With respect to the prior art tuner of the construction shown in FIG. 2, although the spindle 21, gears 19' and 20' and conical block 25 may be integrally formed with each other if the outer diameter of the gear 20' is made smaller than the width of the slot 14b in a manner substantially contrary to that shown in FIG. 2, a similar disadvantage as to the durability of the tuner as a whole still exists in the construction of FIG. 2 because of the sliding engagement between the outer peripheral edge of the boss 15'a and the tapering surface of the conical block 25.